In the field of the driving of motors with alternating currents, whether of synchronous or asynchronous motor type, vector control is commonly used.
Vector control is a generic term denoting all the controls that take real time account of the equations of the system that it controls. The name of these controls stems from the fact that the final relationship is vectorial as opposed to scalar controls. The relationships thus obtained are much more complex than those of scalar controls, but, on the other hand, they make it possible to obtain better performance levels in transient regimes.
For an alternating current motor, there is an operating limit. This is the limitation on the power which is imposed by the power source. In effect, a current supply is produced from a current-regulated voltage source. This source is imperfect and it is limited in power by the voltage of the DC power supply bus.
In the most basic mode of operation of vector control, where the autopiloting angle is kept constant at 0, there is a limit speed, called basic speed, beyond which it is no longer possible to maintain the maximum current (and therefore the maximum torque) because of the voltage limitation of the power source. The difference between this limit voltage and the electromotive force (known also by its abbreviation emf) of the machine in fact becomes insufficient to continue to operate at maximum current.
Beyond the basic speed, control of the regulation loops is lost.
Beyond the basic speed, if there is a desire to continue to control the torque of the machine, it is necessary to deflux (or de-excite) as for direct current machines. The maximum operating power, limited by the power source, is then kept constant and the maximum torque decreases.
This operation is called operating in overspeed mode and this is obtained by defluxing.
The current defluxing techniques are based on control laws derived from the equations of the parameters of the machine. This type of operation presents two major drawbacks. First of all, it is necessary to know the physical parameters of the machine such as, for example, the direct axis inductance of the armature, the quadrature axis inductance of the armature, the resistance of the armature, etc. Furthermore, the machines may be required to operate in non-linear zones in which the equations of the control laws are no longer necessarily borne out.
The knowledge of the parameters of the machine is not always obvious because the value of the parameters can vary as a function of the frequency of use of the machine, of the temperature and of the saturation of the electromagnetic materials.
Furthermore, certain parameters of the machine are sometimes held exclusively by the designer of the machine and are not communicated to the manufacturer responsible for producing the motor control.
Since the current defluxing techniques are based on a precise knowledge of the parameters of the machine, it is essential to systematically re-adapt the defluxing control as soon as the motor to be driven is changed.
Currently, once the linear mode control laws are established, an experimental phase is necessary to be able to weight the parameters of the machine as a function of different physical parameters such as speed of rotation, temperature and saturation of the machine. In other words, in addition to re-adapting the defluxing control to each new machine, it is also necessary to perform additional experimental tests to adapt the parameters.
The defluxing can easily be done on a wound inductor machine. On the other hand, in the case of permanent magnet machines, the defluxing is obtained by injecting currents generating a demagnetizing field.
In the latter case, the effectiveness of the defluxing can be obtained only by virtue of sophisticated controls based on a precise knowledge of the parameters of the machine which will make it possible, through equations, to implement the control laws of the motor.